Rocketing oil prices are being cited as a strong reason for electrifying the Auckland rail network instead of buying new diesel trains for a lower initial cost.

Auckland Regional Transport Authority chief executive Alan Thompson has acknowledged uncertainty about the future availability of power supplies amid controversy over plans to build new transmission lines from the central North Island.

But he is warning members of his organisation's parent body, the Auckland Regional Council, of a greater perceived risk in relying on diesel to fuel a new rail fleet.

"A much more dramatic issue for us is the way oil is going up and up and up," he told the council's transport policy committee, which is considering a recommendation by his board to electrify the network for about $500 million within six years.

"It does seem to us that Auckland, with the rest of New Zealand, has a better opportunity to manage its risks with an electric system than with diesel."

Papers issued by the transport authority this week show that the initial cost of buying 39 new two-car diesel trains to replace elderly rolling stock would be $119 million less than switching to electric.

An estimated cost of $4.25 million for each diesel carriage is, in fact, $540,000 more than for an electric version, and operating and maintenance bills would also be higher.

But it would cost the authority about $121 million to erect overhead wires from Swanson to Papakura or Drury, and a further $44 million to protect track signals from electric fields.

Even so, the authority believes electrification would work out cheaper in the long run at the much higher levels of patronage for which it is aiming over the next 25 to 30 years.

It hopes to boost annual patronage from just under 4 million passenger trips now to 38 million by 2030 with the help of a loop tunnel from Britomart to Mt Eden. If rail were extended to the airport, and from Onehunga to Avondale to complete an outer isthmus loop, patronage could eventually run close to 68 million.

Mr Thompson told the Herald that the point at which electric would become more economically viable than diesel was estimated at 20 million annual trips, but this did not take into account environmental benefits from doing away with fumes and reducing noise.

He added that the figure would drop as the price of oil kept rising.

A rail rapid-transit plan the transport authority made public after considering 11 volumes of technical studies on both motive options for rail notes that electric trains can accelerate faster out of stations than diesel units.